Problem: $ B = \left[\begin{array}{rr}-2 & 0 \\ 1 & -1\end{array}\right]$ What are the dimensions of matrix $ B$ ?
Explanation: The first dimension is the number of rows in the matrix. How many rows does $ B$ have? The second dimension is the number of columns in the matrix. How many columns does $ B$ have? So, $ B$ is a 2 $\times$ 2 matrix.